1. Field of the Invention
This invention relates to an improvement in ultrasonic developing apparatus for imaging an internal image of an object by utilizing ultrasonic waves, and particularly to an improvement in an ultrasonic imaging apparatus which transmits ultrasonic waves to an object and receives the reflected waves or transmitted waves returning from the object.
2. Description of the Prior Art
Ultrasonic imaging technology is widely employed for obtaining an internal image of an object such as a human body.
In such technology, an ultrasonic wave is transmitted to an object, the reflected wave corresponding to the transmitted wave returning from the object is received, and then a tomographic image indicating tissues of an object is obtained on the basis of such a received wave. The ultrasonic imaging has advantages in that it is nondestructive and results in less danger as compared with the existing X-ray imaging.
In general, a widely known ultrasonic imaging apparatus utilizes the pulse reflection method which uses the reflected wave, and therefore the operation of such an ultrasonic imaging apparatus will be explained focusing on this pulse reflection method as an example. The ultrasonic wave pulse of about 1 MHz to 10 MHz is transmitted to an object from the piezoelectric element of a transducer, the echo pulse (reflected wave), which is reflected by a mismatching of impedance at the boundary of two tissues having different acoustic impedances within said object, is received, and the location information of said mismatching area can be obtained by displaying such a received wave. This is called the A mode operation.
A tomographic image can be obtained by the following steps. Namely, the ultrasonic wave pulse transmission location or angle is sequentially shifted according to the location of the tomographic area, and some pieces of location information at the mismatching areas are used for a display based on the data from the reflected waves of respective transmitted pulses. This is called the B mode operation.
In such an existing ultrasonic image apparatus, since the next ultrasonic wave pulse is transmitted after the sufficient time for the preceding ultrasonic wave pulse to be transmitted and then to return as the echo pulse, the period between transmission of ultrasonic wave pulses is resultingly limited to the period required for completing reception of the echo pulse.
Therefore, since the propagative velocity of ultrasonic waves in the tissues of a human body is about 1,500 m/sec, the period from transmission of pulse to reception of echo becomes 2L/1500 (sec) when the depth of boundary is L (meters), and the minimum pulse transmission period is 2L/1500 (sec).
Namely, the number of scanning lines obtained during a second is limited to 1500/2L (lines). For example, when L is 0.2 m, the number scanning lines is 3750 (lines) and therefore it is no longer possible to obtain sufficient scanning lines when a tomographic image is required within a very short period (for example, 0.1 sec).
Moreover, limitation on the number of scanning lines influences the display of a tomographic image on the CRT display unit.
Namely, since the frame rate of about 30 frame/sec is necessary for displaying the motion within an object without flickering, the number of scanning lines in one frame is limited to 25/L (lines). In other words, when L=0.2 m, the number of scanning lines becomes 125 lines/frame which is only 1/4 of the scanning lines in television receivers. Therefore, such limitation on the number of scanning lines results in very rough display of a tomographic image, thereby reducing the image's effectiveness.
In addition, the existing ultrasonic wave imaging apparatus has the disadvantage that a sufficient number of scanning lines cannot be obtained; it is difficult to accurately image momentary conditions of dynamic tissues such as the heart, etc., of a human body and only a very rough display of a tomographic image can be obtained, due to the inherent limitation on the propagation velocity of ultrasonic waves. It has been a necessity for such apparatus to eliminate this disadvantage.
In order to solve such problems, two of the inventors of the present invention have proposed an improved ultrasonic imaging apparatus by the specification of the international patent application No. PCT/JP80/00015 titled "Ultrasonic Diagnostic System", which corresponds to U.S. application Ser. No. 209,403, filed Sept. 22, 1980.
According to this specification, the ultrasonic waves having different frequencies are simultaneously transmitted from a plurality of transducers, the reflected waves or transmitted waves reflecting from an object are received, and then those reflected waves or transmitted waves having specific, predetermined frequencies can be obtained from the total received signal using an electrical filter.
This technique presents simultaneously several times the internal information of the object as that of existing apparatus.
When a human body is selected as an object, attenuation in the object is generally high as the frequency becomes high.
Therefore, when employing the abovementioned technique, it is desirable that differences between plural frequencies are as small as possible, namely that the frequency f.sub.1 of the one transmitting wave and the frequency f.sub.2 of the other wave are very close.
However, when the frequencies f.sub.1 and f.sub.2 are very close, it is difficult to separate them sufficiently using only an electrical filter.
For this reason, even after the received signal has passed an electrical filter, so-called crosstalk occurs. In other words, the signal of the other frequency f.sub.2 still remains in the signal of one frequency f.sub.1.